Technique for sensing the rotational speed and angular position of a rotating wheel using a variable threshold

ABSTRACT

To detect the rotational speed and angular position of a rotating wheel, a non-contact sensor (e.g., an optical sensor or a Hall sensor) scans scan marks on the wheel, and generates a pulse train. The amplitude of the pulses is compared in a comparator with a variable switching threshold. To achieve accurate measurement results, and to compensate for offset and long-term drift of the sensor, the switching threshold is adjusted if one or more of the following conditions is met: (i) the difference between the pulse amplitude and the switching threshold exceeds a fixable first maximum, (ii) the difference of the amplitudes of two successive pulses exceeds a fixable second maximum, (iii) the difference of the frequencies of successive pulses exceeds a fixable third maximum. The method is particularly advantageous in a motor vehicle, to detect the rotational speed and angular position for an electronic ignition system, or the rotational speed and angular position of the wheels for an ABS braking system, an anti-skid system, or a vehicle stabilization system.

BACKGROUND OF THE INVENTION

The invention relates to a technique for detecting the rotational speed(e.g., RPMs) and the angular position of a rotating wheel with anon-contact sensing device.

For electronic control systems in motor vehicles (e.g., an ABS brakingsystem or an electronic ignition system) it is necessary to measureparameters such as rotational speed, the relative angular position of arotating wheel or the crankshaft and/or their angular acceleration orinstantaneous angular velocity. This is often performed by a non-contactsensor (e.g., an optical sensor or a Hall sensor) that scans therotating wheel or a wheel seated on the crankshaft. The rotating wheeltypically includes scan marks (e.g., lines or teeth).

The accuracy of the rotational speed and angular position measurement isoften impaired by external noise signals and/or thermal noise. In ameasurement of angular position, the thermal noise is often noticeableas jitter. If a digital measurement system is used to measure therotational speed and the angular position, there is another error sourcein addition to the thermal noise that garbles the measurement result.This error source is determined by the minimum digital resolution of theleast significant bit (LSB) or by the clock pulse rate. Usually themeasurement error caused by the minimum digital resolution of the LSB isgreater than the measurement error caused by thermal noise. For example,if the position of the threshold changes within one revolution, thescanning times between the individual scan marks on the wheel alsochange, so that the valuation device sees an instantaneous change of therotational velocity, although the rotational velocity has not changed.

Other error sources are the offset of the sensor and the long-term driftdue to aging of the sensor, or a gradual change of temperature in themeasurement environment (e.g., by the warming of the engine).

Therefore, there is a need for a technique for compensating for variouserror sources associated with sensing rotational speed and/or angularposition.

SUMMARY OF THE INVENTION

An object of the present invention is to process a pulse train outputsignal provided by a non-contact sensor that senses the rotational speedand angular position of a rotating wheel, in such a way that the offsetand long-term drift errors due to aging of the sensor or the offseterror due to temperature change are compensated for accuracy.

To detect the rotational speed and the angular position of the wheel,the non-contact sensor (e.g., an optical sensor or a Hall sensor) scansmarks on the wheel, and generates a pulse train indicative of therotational speed and angular position of the wheel. The amplitude of thepulses is compared in a comparator with a variable switching threshold.To compensate for offset and long-term drift of the sensor, the variableswitching threshold is adjusted based upon one or more characteristicsof the pulse train (i.e., threshold adjustment criteria).

In one embodiment, the amplitude of the pulses from the sensor arecompared in the comparator with the variable switching threshold value,which can be adjusted so the difference between the amplitudes of thepulses and the switching threshold does not exceed a fixable firstmaximum value. For example, when the difference between the pulseamplitude and the switching threshold of the comparator exceeds thefixable first maximum value, the switching threshold is adjusted so thatthis difference falls below the fixable first value. If the amplitude ofthe pulses generated by the sensor is greater than the switchingthreshold value, the switching threshold value is increased; on theother hand, the value of the switching threshold is decreased if theamplitude of the pulses is less than the switching threshold value.

In another embodiment, the positive and negative amplitude of the pulsesfrom the sensor, or their maxima and minima, are compared in thecomparator with the variable switching threshold, which is adjusted sothat the difference between the extremes or the amplitudes of the pulsesand the variable switching threshold does not exceed a fixable secondmaximum value.

In yet another embodiment, the pulses from the sensor are compared inthe comparator with the variable switching threshold, which is adjustedif the difference between the amplitudes of two successive pulsesexceeds a fixable third maximum value. For example, if the evaluationcircuit determines that the amplitude of the second pulse is greaterthan that of the first pulse, the switching threshold is increased.Initially, if the amplitude of the second pulse is less than that of thefirst pulse, the value of the switching threshold is decreased.

In still another embodiment, the amplitude of the pulses is compared inthe comparator with the variable switching threshold, which is adjustedif the difference of the frequencies of successive pulse trains exceedsa fixable fourth maximum value.

The variable switching threshold is preferably positioned by theevaluation circuit as stably as possible and is situated in the middlebetween the minimum and maximum sensor value.

The threshold adjustment techniques of the present invention may be usedalone or in combination with one another. In addition, the switchingthreshold of the comparator is preferably only adjusted during thesynchronization signal. It is contemplated that in some embodiments, thevarious individual threshold adjustment criteria may be combined withone another in order to define the criteria when the evaluation circuitmay adjust the threshold. For example, consider one embodiment where thefollowing conditions must be fulfilled simultaneously in order for theevaluation circuit to adjust the threshold:

-   -   1. The difference between the pulse amplitudes and the switching        threshold exceeds a first fixable threshold value.    -   2. The difference of the amplitudes of successive pulses exceeds        a second fixable maximum.    -   3. The difference of the frequencies of successive pulses        exceeds the third fixable maximum.    -   4. The RPM of the wheel is greater than a fixable minimum.    -   5. A synchronization signal is generated.

Advantageously, inventive adjustment of the switching threshold improvesthe accuracy of the measurement. Furthermore, the offset and long-termdrift of the sensor due to aging are largely compensated for.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a rotational sensing system;

FIG. 2 illustrates an alternative embodiment rotational sensing system;and

FIG. 3 illustrates yet another alternative embodiment rotational sensingsystem.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a rotational sensor system 10, which includes asensor 12 that reads scan marks 14 on a rotating wheel 16, and providesa sensor output signal on a line 17. The sensor output signal is inputto a comparator 18, which also receives a first threshold signal on aline 20, and provides a rotational sensing output signal on a line 22.

The sensor scans the scan marks on the wheel. These scan marks can belines that are scanned (i.e., read) by an optical sensor, or they can beteeth that are scanned by a Hall sensor. The synchronization mark can bea broad line, a missing tooth, or a broad tooth. When scanning a scanmark—a line or a tooth or a tooth gap, the sensor generates a pulse orskips a pulse, for example in the case of a tooth gap. The data read bythe sensor are input to an evaluation circuit that determines therelative angular position of the wheel, its instantaneous angularvelocity, and/or its angular acceleration.

According to an aspect of the present invention, an evaluation circuit24 receives the sensor output signal on the line 17 and compares theamplitude(s) of the pulses generated by the sensor 12 against the firstthreshold signal value on the line 20. If the difference between theamplitude of the pulses and the first threshold signal on the line 20exceeds a fixable first maximum value, the evaluation circuit 24 adjuststhe value of the first threshold signal on the line 20. For example,when the difference between the pulse amplitude and the switchingthreshold of the comparator exceeds the fixable first maximum value, thethreshold signal on the line 20 is adjusted so that this differenceagain falls below the fixable first value. If the amplitude of thepulses generated by the sensor is greater than the threshold value onthe line, the switching threshold value is increased; on the other hand,the value of the threshold signal on the line 20 is decreased if theamplitude of the pulses is less than the current value of the threshold.

In another embodiment, the positive and negative amplitude of the pulsesfrom the sensor, or their maxima and minima, are compared in thecomparator with a variable switching threshold. In this embodiment thevariable switching threshold on the line 20 is adjusted so thedifference between the amplitudes of the pulses and the variableswitching threshold does not exceed a fixable second maximum value.

The evaluation circuit 24 may also be configured to compare theamplitudes of successive pulses of the sensor output signal on the line17. If the difference of the amplitudes of the two successive pulsesexceeds a fixable third maximum value, the evaluation circuit 24 adjuststhe value of the switching threshold on the line 20. For example, if theevaluation circuit determines that the amplitude of the second pulse isgreater than that of the first pulse, the switching threshold isincreased. Initially, if the amplitude of the second pulse is less thanthat of the first pulse, the value of the switching threshold isdecreased.

In yet another embodiment, the evaluation circuit 24 adjusts thecomparator threshold if the difference of the frequencies of successivepulses exceeds a fixable fourth maximum value.

The evaluation circuit 24 may be configured to apply any of theseforegoing threshold adjustment criteria either alone or in combination.

FIG. 2 illustrates an alternative embodiment rotational sensing system30. This system also includes the sensor 12 that cooperates with therotating wheel 16, and provides the sensor output signal on the line 17.The system illustrated in FIG. 2 is substantially similar to the systemdisclosed in FIG. 1, with a principal exception that the sensing system30 illustrated in FIG. 2 includes a synchronization detector 32. Thesynchronization detector 32 receives the output signal on the line 22and provides a sync output signal on a line 34, which is input to theevaluation circuit 24. In this embodiment, the evaluation circuit 24 isonly allowed to make changes to the threshold signal on the line 20 whenthe synchronization signal on the line 34 is valid, indicating thepresence of the synchronization signal. When the synchronization signalis detected, the evaluation circuit 24 is then allowed to make theadjustments as set forth above. For example, if the evaluation circuit24 is configured to adjust the threshold when the difference between theamplitude of the pulses and the threshold signal on the line 20 exceedsthe fixable first maximum value, this adjustment will only be made whenthe sync signal on the line 34 is valid. That is, if the sync signal onthe line 34 is not valid, the threshold signal will not be adjusted evenif the difference between the amplitude of the pulses and the thresholdsignal on the line 20 exceeds the fixable first maximum value. Ofcourse, any of the adjustment criteria set forth above may be used aloneor in combination in the system illustrated in FIG. 2.

FIG. 3 illustrates yet another alternative embodiment rotational sensingsystem 50. This embodiment, is substantially the same as the embodimentillustrated in FIG. 2, with the principal exception that the embodimentin FIG. 3 includes a filter 52, that receives the sensor output signalon the line 17. The circuit arrangement shown in FIG. 3 may adjust theswitching threshold in accordance with one or more of the specifiedthreshold adjustment criteria.

The invention is especially suited for use in a motor vehicle, where,for example, it accurately measures: (i) the rotational speed andangular position of the crankshaft for an electronic ignition system, or(ii) the rotational speed and the angular position of the individualwheels for an ABS braking system, an anti-skid control, or a vehiclestabilization system.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. A method of detecting rotational speed and angular position of arotating wheel with a non-contact sensor that provides a pulse trainoutput signal, said method comprising: comparing the amplitude of thepulses of the pulse train output signal with a first variable switchingthreshold value to provide an output signal; receiving the output signaldetect a synchronization signal and provide a sync signal indicativethereof; and adjusting said switching threshold value when thedifference between the amplitudes of the pulses and said switchingthreshold value exceeds a fixable first maximum value and said syncsignal is valid.
 2. The method of claim 1, wherein said switchingthreshold is adjusted if the difference between the amplitudes of thepulses and said switching threshold exceeds said fixable first maximumvalue, and at the same time the difference between the extremes or theamplitudes and said switching threshold exceeds a fixable second maximumvalue.
 3. The method of claim 1, wherein said switching threshold isadjusted if the difference between the amplitudes of the pulses and saidswitching threshold exceeds said fixable first maximum value, and at thesame time the difference of the amplitudes of two successive pulsesexceeds a fixable second maximum value.
 4. The method of claim 1,wherein said switching threshold is adjusted if the difference betweenthe amplitudes of the pulses and said switching threshold exceeds saidfixable first maximum value, and at the same time the difference offrequencies of successive pulses exceeds a fixable second maximum. 5.The method of-claim 1, wherein said switching threshold is adjusted ifthe difference between the amplitudes of the pulses and said switchingthreshold exceeds the fixable first maximum, at the same time thedifference between the extremes or the amplitudes of the pulses and thevariable switching threshold exceeds a fixable second maximum, and atthe same time the difference between the amplitudes of two successivepulses exceeds a fixable third maximum.
 6. The method of claim 1,wherein said switching threshold is adjusted if the difference betweenthe amplitudes of the pulses and said switching threshold exceeds saidfixable first maximum and at the same time the difference between theextremes or the amplitudes the variable switching threshold exceeds afixable second maximum, and at the same time difference of thefrequencies of successive pulses exceeds a fixable third maximum.
 7. Themethod of claim 1, wherein said switching threshold is adjusted if thedifference between the amplitudes of the pulses and said switchingthreshold exceeds said fixable first maximum, and at the same time thedifference of the amplitudes of two successive pulses exceeds a fixablesecond maximum, and at the same time the difference of the frequenciesof successive pulses exceeds a fixable third maximum.
 8. The method ofclaim 1, wherein said switching threshold is adjusted if the differencebetween the amplitudes and the pulses and said switching thresholdexceeds said fixable first maximum value, and at the same time thedifference between the extremes or the amplitudes of the pulses and thevariable switching threshold exceeds a fixable second maximum value, andat the same time the difference between the amplitude of two successivepulses exceeds a fixable third maximum value, and at the same time thedifference of the frequencies of successive pulses exceeds a fixablefourth maximum value.
 9. The method of claim 8, comprising an evaluationcircuit that receives said pulses and determines the relative angularposition of the wheel and its instantaneous rotational velocity, andprovides signals indicative thereof.
 10. The method of claim 8, whereinthe value of said switching threshold is increased if the difference ofthe amplitudes has a positive sign, and the value of said switchingthreshold is lowered if the difference signal has a negative sign. 11.The method of claim 8, wherein the value of said switching threshold isincreased if the difference of the frequencies has a positive sign, andis lowered if the difference of the frequencies has a negative sign. 12.The method of claim 8, comprising the step of enabling the adjustment ofsaid threshold signal if a received synchronization signal is valid. 13.A method of detecting rotational speed and angular position of arotating wheel with a non-contact sensor that provides a pulse trainoutput signal, said method comprising: comparing the amplitude of thepulses of the pulse train output signal with a variable threshold valueto provide an output signal; receiving the output signal to detect asynchronization signal and provide a sync signal indicative thereof; andadjusting said variable threshold value to maintain the differencebetween the amplitudes of the pulses an said variable threshold valueless than a first value when said sync signal is valid.
 14. A method ofdetecting rotational speed of a rotating wheel with a non-contact sensorthat provides a pulse train output signal, said method comprising:comparing the amplitude of the pulses of the pulse train output signalwith a variable threshold value to provide an output signal indicativeof rotational speed; receiving the output signal to detect asynchronization signal and provide a sync signal indicative thereof; andadjusting said variable threshold value to maintain the differencebetween the amplitudes of the pulses and said variable threshold valueless than a first value when said sync signal is valid.